Odd enemy AI behavior

In my current game I've noticed some rather strange enemy AI behavior that I'm not sure if it's supposed to be normal or if it's a bug. Sometimes an enemy will just stand there and not attack. Other times it will move around but still not attack me. Lastly groups of monsters will appear out of nowhere just beyond my line of sight in a spot I just walked through. Has anyone else noticed anything like this happening in their games?
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